Field of the invention
The present invention relates to a control system for optical disc information reproducing apparatus, and more particularly to a control system for a moving device such as disc rotation drive motors, pickup transfer drive motors, pickup lens focusing or tracking actuators of optical disc players such as digital audio disc plyers, video disc players or memory disc apparatus.
Description of the Prior Art
There have been provided optical record discs which store optically recorded information data or signals such as digitized audio signals, video or image signals and any other information data and optical record disc reproducing apparatus for reproducing the recorded information data. The optical record discs of such type, for example, compact discs (CD) (a type of digital audio discs), optical video discs, optical image discs and the like are so arranged that digital data intended for high density recording are recorded in the form of pit strings forming a spiral track or plurality of concentric tracks on one side of the optical disc and reproduced through reading the pit strings by means of transducer means like an optical pickup device using laser beams during a rotation of the optical record disc at a predetermined velocity.
The disc reproducing apparatus are provided with a rotation drive motor for rotating the optical disc at the predetermined velocity. In the compact disc players, for example, the rotation drive motor drives compact discs at a constant linear velocity (CLV). While, the rotation drive motor of the video disc players drives video discs at a constant angular velocity (CAV). The rotation velocities of the disc rotation drive motors are controlled to keep the CLV or CAV velocity in constant in the disc reproducing operation. The rotation velocity can be maintained to keep the predetermined CLV or CAV state with a relatively low power consumption during the normal disc reproducing operation. There, however, is required a relatively large power consumption for bringing the disc rotation drive motors from a stop state to the predetermined velocity state or vice versa at a start to the disc reproducing operation or at an interruption to a reproducing stop state.
The disc reproducing apparatus also is provided with a pickup transfer drive motor for transferring the optical pickup in the radial direction of the optical discs. The transfer speed of the pickup transfer drive motor is controlled at a relatively low speed in the disc reproducing operation. Therefor the transfer speed at that time can be maintained in constant with a relatively lower power consumption. While the optical pickup device is transferred at a relatively high speed from one position to another position at a long distance track search operation, the reproducing start operation and the reproducing stop operation. At the operations, there is required a relatively large power consumption for bringing the pickup transfer drive motor from the low speed transfer state to the high speed transfer state or vice versa.
The disc reproducing apparatus also is provided in the optical pickup device a lens, a focusing actuator and a tracking actuator. The lens places the laser beam on the record disc. The focusing actuator drives the lens in the perpendicular direction of the optical disc or the longitudinal direction of the light beam for controlling a focus of the laser beam on the optical disc. The tracking actuator drives the lens in the radial direction of the optical disc or the perpendicular direction of the light beam for controlling the laser beam to follow the track of the optical disc.
The actuators adjust or shift a location of the lens in a relatively narrow distance range with a relatively low power consumption at the ordinary reproducing operation. While the actuators are required to shift the lens in a relatively wide distance range to make the laser beam focus or track on a given track of the optical disc at the shorts distance track search operation, the reproducing start operation and the reproducing stop operation. At the operations, there is required a relatively large power consumption of the actuators to shift the lens in the wide distance range.
Further the actuators are required to fail for shifting the lens at an occurence of failures of the information data pickup, e.g., signal dropouts in the optical pickup device. At that time, it is required that the actuators are given a power less than the power at the normal disc reproducing operation.
The moving devices such as the disc rotation drive motor or the like are servo controlled by some servo control signal obtained from the reproduced information data through the optical pickup device for keeping the optical disc or the like in a proper state of the rotation velocity or the like during the normal disc reproducing operation. As the servo control signal, a tracking error signal, a focusing error signal or a synchronous control signal such as an automatic frequency control (AFC) signal can be used. While at a prescribed state such as the start operation, the search operation or the like, the moving devices are controlled by some other signal for compelling the prescribed state, such as a start command signal given from an operation control section or a signal indicating an abnormal in the servo control signal. Then a power supply for the moving devices are controlled by the servo control signal or the compelling signal.
By the way, a demand for a portable use has increased in the field of compact disc players and video disc players. In the portable disc players, batteries are generally used as a power source for driving the moving devices. As is well known, a problem arises in designing the portable disc players in particular for lowering the power consumption and the power supply voltage as little as possible.
To solve the problem, the power supply for driving the moving devices popularly is performed in the field of the portable disc players by using a PWM (pulse width modulation) technique. The reason is that the PWM power supply has a higher efficiency of a power conversion from the electric energy to the kinetic energy.
However, in a conventional disc reproducing apparatus, particularly in a portable disc reproducing apparatus using the PWM power supply as described above, the following problem arises in respect of the power conversion efficiency at driving loads (moving devices).
However, in a conventional compact disc player using the PWM technology as described above, the following problem arises in respect of the efficiency at driving the load (pickup drive motor) by a pulse width modulation signal, that is, in respect of the conversion efficiency from electric energy to kinetic energy.
First, an electric power W at driving the load by a pulse width modulation (PWM) signal is expressed by the following equation: EQU W=E.sup.2 / R (1)
where R is the resistance of the load, and E is the effective voltage of the PWM signal. FIG. 1 is a graph showing the waveform of the PWM signal.
When taking a cycle of the PWM signal as T and an instantaneous voltage as e, the effective voltage E becomes as follows: ##EQU1##
When dividing the instantaneous voltage e into sine wave (high harmonic) components e.sub.n (n is a positive integer, i.e., n=1, 2, 3 . . . ) by Fourier series conversion respectively, the maximum voltage E max.sub.n of the sine wave components e.sub.n becomes as follows: EQU e.sub.n =Emax.sub.n .multidot.sin (n.omega.t+.theta.n) (3)
where .omega. is an angle frequency of the PWM signal.
The effective voltage E becomes as follows. ##EQU2## where, Eo is a direct current (DC) component.
The performing Fourier series conversion of the PWM signal as shown in FIG. 1, the following equation is obtained. ##EQU3##
Substituting .omega.=2.pi./T, the following equation is obtained. ##EQU4##
When obtaining the effective voltage E from above equation, ##EQU5##
In the above equation, only the DC component Eo (Eo=.pi.V/T) of the PWM signal is essentially a voltage to be converted into an effective kinetic energy and other higher harmonic components become ineffective. As a result, the power conversion efficiency can be obtained by obtaining the ratio (Eo/E) of the DC component Eo and the effective voltage E. As will be clear from the equation described above, the effective voltage E is a function of .tau./T, that is, the ratio (hereinafter) called as duty ratio) of the ON time period or the pulse width .tau. of the PWM signal and the cycle T of the PWM signal. It is clear that the power conversion efficiency depends on the duty ratio.
FIG. 2 is a diagram showing the characteristic of the power conversion efficiency to the duty ratio when the PWM signal of the carrier frequency 44.1 kHz at the peak value of 1V is applied to a load with the resistance R. In this case, it is assumed that the pulse width .tau. is varied while the cycle T of the PWM signal is made constant, thereby to change the duty ratio.
In FIG. 2, the solid line shows the actually measured value of the power conversion efficiency characteristic, while the broken line shows the value obtained from theoretical calculation made by dividing the value of the DC component Eo by the effective voltage e obtained from equation (7) for the higher harmonics from the first order (n=1) to the 100th order (n=100).
As is clearly understood from the power conversion efficiency characteristics shown in FIG. 2, it is theoretically and experimentally clarified that the power conversion efficiency is remarkably reduced as the duty ratio is decreased, that is, as the pulse width .tau. is shortened.
This means that for the same effective voltage Ve can be obtained from two kinds of PWM signals PWMa and PWMb, the former PWMa having a larger peak value Va and the latter PWMb having a smaller peak value Vb while the former PWMa having a narrower pulse width .tau.a and the latter having a wider pulse width .tau.b as shown in FIG. 3a and FIG. 3b. However, the power conversion efficiency of the latter PWM signal PWMb shown in FIG. 3b is higher than that of the former PWM signal PWMa shown in FIG. 3a.
Due to the above reason, it is advantageous that a PWM signal having the largest duty ratio if PWM signals with the same effective voltage is used to drive a load, e.g., the moving device like the disc rotation drive motor. However, PWM signals are required that they have a fairly high effective voltage sufficient to make the moving device move at a high speed at the prescribed operation such as the start of the disc reproducing operation as described above. From this purpose, it is also necessary to set the peak value of a PWM signal as high as possible.
PWM signals normally are applied to motors after amplified through some amplifiers. The peak value of the PWM signal is therefore inevitably determined by the power supply voltage applied to the amplifier in the conventional disc reproducing apparatus. In this case, the PWM signal is oblidged to have a relatively low duty ratio in the normal reproducing operation state so that the moving device moves the optical discs with a relatively large power consumption.